There have been a number of proposals to prevent a vehicle from veering off a prescribed driving lane or road owing to the loss of the alertness of the vehicle operator. For instance, Japanese patent laid-open publication (kokai) No. 6-255514 discloses a steering control system which detects the current position of the vehicle relative to a lane dividing line by capturing its image with an image processing device, and controls a power steering system so as to maintain the vehicle centrally in the driving lane.
However, this previously proposed steering control system can detect any intention of the vehicle operator to change the driving lane or to make a right or left turn only by the actuation of a turn signal. Therefore, the system may be able to detect any deviation of the vehicle from a straight path which may be caused unintentionally by the vehicle operator, but is unable to properly respond to any intentional effort made by the vehicle operator to swerve the vehicle away from a straight path. For instance, when the vehicle operator attempts to avoid any object on the road by swerving the vehicle away from its straight course, the steering control system, unable to detect the intention of the vehicle operator, may resist such an effort made by the vehicle operator.
Thus, it has been desired to provide a vehicle steering control system which can distinguish between an unintentional veering of the vehicle away from a proper course of travel and an intentional deviation of the vehicle from an originally intended path of travel.